DMX -- Crashes Alicia Keys Concert ... UP IN HERE!!

DMX did not get arrested last night ... but he did hijack Madison Square Garden -- turning an Alicia Keys concert into an absolute insane asylum ... and the video is AWESOME!!

First, the crowd went berzerk when a mini Ruff Ryders reunion broke out -- Alicia's husband Swizz Beatz and DMX hit the stage for a few bars of "Ruff Ryders Anthem."

But you've gotta see what happens when they launch into "Party Up (Up in Here)."